Position broadcast systems, either terrestrial or satellite based, are used to provide location information to user devices. Examples of such systems include: GPS (Global Positioning System), GLONAS, Galileo, and WAAS or extended GPS.
Some mobile devices are now being made with the ability to obtain position or location information from such systems. For example, many mobile devices have GPS receivers. Applications on such mobile devices or on servers supporting the mobile devices can integrate the position information obtained. For example, a mapping application may use the information to display a map showing a mobile device's current position.
GPS uses a plurality of satellites, presently approximately 30. Devices with GPS receivers can typically see 12 to 16 satellites at any one time. A device needs to be able to see or receive signals from at least three satellites to lock on to a position. To calculate latitude, longitude and altitude, signals from at least four satellites are required. Each satellite continuously broadcasts messages, in the form of packets, containing an identifier, timing signature, ephemeris data and almanac data. Almanac data indicates where in the sky the satellite is located. Ephemeris data contains information to account for distortions in the position and time data.
Strength of signals from GPS satellites at a GPS receiver's location will depend on many factors, such as where in the sky the respective satellite is located, obstacles blocking the signal and atmospheric conditions. Signals from GPS satellites are difficult to receive while the GPS receiver is inside a building because the structure of the building blocks or distorts the signals. Often, when a GPS receiver enters a building it is no longer able to lock onto a position. Sometimes, if the GPS receiver has a lock and it enters a building a lock can be maintained, if only temporarily. However, if the lock is lost, for example, if the device enters an elevator, then it is difficult or impossible to obtain a lock again within the building.
GPS receivers' power consumption is at its highest when searching for satellites for the purpose of locking on to a position. Some user devices have procedures for a cold start, a warm start and a hot start. For a cold start, a user device will instruct its GPS receiver to search for every satellite in the sky. For a warm start, the user device will instruct the GPS receiver to search for satellites from a narrower range of satellites based on the user devices last known position. A hot start, an even narrower range is used. For a cold start uses more power than a warm start or a hot start.
Many devices with GPS receivers will continue to search for satellites when the device is out of range of those signals, for example inside a building. Continuously searching for satellites is very power intensive and can have a dramatic effect on length of time a battery in a mobile device maintains its charge.